This invention relates generally to GaAs mixer diodes and more particularly to a high-frequency, low-barrier-height GaAs mixer diode which can operate at low noise figure levels at low local oscillator power levels (0.25 mW- 0.75 mW).
The Schottky-barrier diode is a rectifying metal-semiconductor junction formed by plating, evaporating, or sputtering a variety of metals on n-type or p-type semiconductor materials. Generally n-type silicon and n-type GaAs are used. Due to higher cutoff frequency, GaAs devices are preferred in applications above X-band frequencies. This results from the higher mobility of electrons in GaAs than in silicon. Although in practice this advantage is not as significant as predicted, conversion loss improvement of 0.5 dB at K, band is readily obtainable with GaAs over silicon.
A mixer diode is a device that produces the conversion of a low power level signal from one frequency to another by combining it with a higher power (local-oscillator) signal in a nonlinear device. In general, mixing produces a large number of sum and difference frequencies. Usually the difference frequency between signal and local oscillator-the intermediate frequency (IF)-is of interest and is at a low power level.
Although GaAs Schottky barrier mixer diodes have demonstrated a low noise figure, the barrier heights of such diodes have been substantially higher than those of most silicon Schottky diodes. Due to the pinning of the Fermi level in metal-GaAs systems, the barrier height is vertually independent of the metal and has a typical value of 0.75-0.80 eV. Since the barrier heights of GaAs mixer diodes require high oscillator power. This has prevented the utilization of GaAs mixer diodes in microwave systems which have low, limited local oscillator power.
Initial attempts for developing low barrier height GaAs mixer diodes utilized AuGe alloy as the barrier metal on n-type epitaxial GaAs. These diodes were fabricated for IN78 point contact packages to operate at Ku-band frequencies. This approach, however, is not practical due to failure upon thermal-compression bonding.
Alternatively, low-barrier Schottky diodes have been developed by implanting germanium into epitaxial GaAs. However, these diodes required encapsulation and high temperature annealing to eliminate ion-implantation damage.